The ability to tab-complete within command blocks was added in snapshot 15w34b (1.9). Command blocks also support running commands as just minecraft:give instead of minecraft:give. However, craftbukkit doesn't understand this, and assumes that it's a chat message.

There is a field in the tab-complete packet that indicates that it the text being tab-completed should always be treated as a command; currently that is ignored. (I'm probably going to be able to fix this myself, just creating a reference issue)